This invention relates to a receiver applicable to loran-C, a well-known hyperbolic navigation technique, and more particularly, a loran-C receiver which tracks a specified cycle of the carrier of the received loran signal pulses.
Loran-C employs a chain of a single master station and two or more secondary stations. The master station generates 9 loran pulses, as shown at M in FIG. 1(a) of the attached drawings while the secondary stations each transmit 8 loran pulses, as shown at S.sub.1, S.sub.2 in FIG. 1(a). These pulse groups of the master and secondary stations are all repeated at the same, fixed frequency. Each of the secondary stations transmits a loran pulse group with a specified time lag (coding delay) from transmission of the loran pulse group from the master station. The coding delay of one secondary station is different from that of the other secondary station of the same chain.
The loran-C receiver finds the difference between the distances from the fixed positions of the master and secondary stations on the basis of the reception time lags of the secondary station pulses relative to the master station pulses, and identifies the position of the receiver from two hyperbolic curves between the master station and each of the secondary stations. In order to find the reception time lag of the secondary master pulses with respect to the master station pulses, the receiver also locates a specified cycle (generally, the third cycle) of the carrier of each of the received station pulses and automatically tracks the specified cycle.
The carrier Ca of the loran pulses, as shown in FIGS. 1(b) and 1(c) which show FIG. 1(a) on a progressively expanded time scale, has a frequency of 100 KHz and a period of 10 .mu.sec.
One prior art loran-C receiver having the above-mentioned functions is disclosed in examined Japanese patent publication No. 56-2312, published on Jan. 19, 1981. This receiver uses a pair of sample pulses P.sub.1, P.sub.2 separated by 2.5 .mu.sec, as shown in FIG. 2(b) to locate the third cycle of the loran pulse carrier Ca, as follows: The period of the pulses P.sub.1, P.sub.2 matches the repetition period of the loran pulses LP (in the Japanese maritime province, 99.7 msec), and the pulses P.sub.1, P.sub.2 are shifted backwards in phase toward the leading edge of the loran pulse LP in steps of one period (10 .mu.sec) of carrier Ca from the point at which the sample pulses are synchronized with the pulse LP.
After the sample pulses P.sub.1, P.sub.2 no longer coincide with loran pulse LP, the sample pulse pair is shifted in additional 30 .mu.sec backwards so that they precede the loran pulse LP by at least 30 .mu.s, as shown in FIG. 2(b). Then, as shown in FIG. 2(c), the direction of movement of the sample pulse (P.sub.1, P.sub.2) pair is reversed and the sample pulse pair is shifted stepwise toward the loran pulse LP in steps of 10 .mu.sec. After the pulse pair again reaches the leading edge of loran pulse LP, which allows the position of the third cycle of the carrier Ca (about 30 .mu.sec backward of the leading edge of loran pulse LP) to be recognized, the tracking of the third cycle begins.
In this receiver, each time the sample pulse pair (P.sub.1, P.sub.2) is moved by one step, the presence of carrier Ca is checked, so that a plurality of samples of the received signal must be taken. In order to improve the S/N ratio of the received loran pulses LP, the number of samples of the received signal taken after each shift of the sample pulse pair should be maximized. When the S/N ratio is 0 dB or less, measurement must be taken tens to hundreds of times to ensure accurate readings.
This prior art receiver, however, uses only one pair of sample pulses, so that it takes a long time to locate the third cycle of carrier Ca. Furthermore, when the first detected loran pulse LP is a space wave of the loran signal reflected by the ionized layers of the atmosphere, it takes a longer time to detect the third carrier cycle: the space wave has a time lag of about 40--hundreds of .mu.sec with respect to the surface wave of the loran signal. The space wave is less attenuated during propagation than the surface wave, so that it has a higher intensity at the receiver than the surface wave. If the sample pulse pair P.sub.1, P.sub.2 is initially synchronized with the space wave of the loran signal at a point, for example, 300 .mu.sec after the leading edge of the surface wave, 30 or more steps of the sample pulse pair and tenshundreds of measurements of the received signal for each step would be required to locate the leading edge of the surface wave and thus it would take tens of seconds--several minutes to locate the third cycle of the carrier.
In addition, if the position of a moving vehicle with a loran receiver such as mentioned above is desired to be measured, the problem mentioned above can not be neglected since the surface wave is greatly attenuated in city streets or inter-mountain areas, and hence the S/N ratio of the received signal tends to be less than 0 dB.
Moreover, when the vehicle travels through areas such as tunnels where the loran-C electromagnetic waves are not available, the sample pulses will lose synchronization with the loran pulse carrier, so that it may take tens of seconds to several minutes until position measurement again starts after the vehicle has passed through a tunnel, which would be very inconvenient.